Non-Patent Document 1 and Patent Documents 1 and 2 disclose magnetic-resonance isolators. Such magnetic-resonance isolators of the related art utilize a phenomenon in which when high-frequency currents of equal amplitude whose phases differ by π/2 radians flow in two perpendicular lines, a rotating magnetic field (circularly polarized wave) is produced at the intersection thereof and the rotational direction of the circularly polarized wave reverses depending on the traveling direction of the electromagnetic wave along the two lines. Specifically, a ferrimagnetic member is disposed at the intersection, and a static magnetic field needed for magnetic resonance is applied. When the traveling direction of the electromagnetic wave propagating in the principal line is the reverse direction, the circularly polarized wave produced at the intersection is a positive circularly polarized wave, and resonance absorption occurs. When the direction of the electromagnetic wave propagating in the principal line is the forward direction, the circularly polarized wave is a negative circularly polarized wave, and resonance absorption does not occur so that the electromagnetic wave can be transmitted.
FIG. 28 illustrates the structure disclosed in Non-Patent Document 1. In the example shown in FIG. 28, lines composed of conductor layers 6a, 6b, and 6c are held from the upper and lower sides thereof between dielectric substrates 1a and 1b each having a shield electrode 7 to form a balanced strip line, and a cross-shaped λ/4 resonator is defined in the conductor layer 6a. A circularly polarized wave is produced at the intersection of the resonator and the principal line extending in the horizontal direction, and the rotational direction of the circularly polarized wave changes in the forward or reverse direction depending on the traveling direction of the electromagnetic wave propagating in the principal line. By applying a static magnetic field needed for magnetic resonance to a ferrite core 16, for example, in the case of a positive circularly polarized wave, resonance absorption occurs, and, in the case of a negative circularly polarized wave, absorption does not occur and the electromagnetic wave is transmitted. This arrangement acts as an isolator.
FIG. 29 illustrates the structure of the isolator disclosed in Patent Document 1. In the example shown in FIG. 29, a ferrite core 16 is disposed in the central portion of a dielectric plate 1, and a bonded conductor 17 having four ports perpendicular to each other is disposed on the top of the ferrite core 16. One of two opposed ports of the four ports is provided with a lumped-constant capacitor 19, and the other port is provided with a lumped-constant inductor 20. The remaining opposed ports serve as input/output terminals 18.
FIG. 30 illustrates the structure of the nonreciprocal circuit device disclosed in Patent Document 2. In the example shown in FIG. 30, a disk-shaped ferrite core 16 is embedded in the central portion of a corner-shaped dielectric plate 1. On the upper surface of the electric plate 1, matching circuits 18a and 18b are disposed in four port of a bonded conductor 17, with the ends thereof being used as input/output terminals. The two remaining ports are provided with lines 18c and 18d that are connected with open-end lines configured such that lines 18c′ and 18d′ are defined on dielectric plates 1′ and 1′.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 63-260201    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-326504    Non-Patent Document 1: Tadashi Hashimoto, “Maikuroha Feraito to sono Oyo Gijutsu (Microwave Ferrite and its Applied Technology)”, the first edition, Sogo Denshi Shuppansha, May 10, 1997, pp. 83-84
Neither of Patent Document 1 or 2 or Non-Patent Document 1 discloses a substantially cross-shaped strip-line resonance isolator that is formed by intersecting microstrip lines. The facts that the fundamental mode is a dual mode and that the magnetic field vectors are orthogonal to each other in the vicinity of the intersection, i.e., that a circularly polarized wave is produced at a certain frequency, are utilized to form a magnetic-resonance isolator. However, such a nonreciprocal circuit device of the related art is designed to operate at a half wavelength or a quarter wavelength because of the use of microstrip lines. It is difficult to reduce the size because the pattern size is determined based on the dielectric constant of the substrate. Further, the magnetic field distribution is of the distributed-constant type, and a region in which a circularly polarized wave having the magnetic resonance absorption effect is produced is also of the distributed-constant type. Thus, the absorption efficiency with respect to the volume of a magnetic-material member is low, and it is also difficult to reduce the size of the magnetic-material member.
In a microstrip-line resonator composed of a nonreciprocal circuit device of the related art, the magnetic field vectors are expanded to the outside in which no microstrip-line electrodes exist. This limits the compactness and integration of the circuit.